The present invention relates to apparatus for the plasticizing and melting of synthetic plastic materials, especially granulae, wherein the plasticizing and melting operations take place in the interior of a heated cylinder or barrel for an elongated rotary and reciprocable feed screw. Such apparatus may be used for injection of plastic material into the mold or molds of an injection molding machine.
Apparatus for plasticizing and melting of granular synthetic plastic materials include a feeding unit which admits granulae to one end of the plasticizing cylinder whereby the material undergoes compression, heating and plasticizing as a result of rotation of the feed screw as well as owing to heating of the cylinder. The conversion of granular starting material into a mass which is ready to be converted into injection molded products is accompanied by release of volatile constituents which must be segregated from the plasticized material before the latter issues from the nozzle or die at the forward or discharge end of the cylinder. The development of water vapors in the plasticizing cylinder can be prevented, or its extent reduced, by drying the granular material prior to introduction into the apparatus. A drawback of drying prior to admission into the cylinder is that the initial and maintenance cost of drying equipment is quite high. Moreover, the operation of such equipment must be monitored and regulated with a high degree of accuracy because, if not properly conditioned, the stream or streams of gaseous fluid (normally air) which are utilized for drying of granular material prior to admission into the plasticizing cylinder are likely to cause baking and conversion of granulae into clumps or similar agglomerations.
It is already known to provide the plasticizing the cylinder with an aperture which permits the escape of volatile constitutents from the plasticized synthetic plastic material while such material advances toward the nozzle or die at the forward end of the cylinder. It is also known to place such aperture downstream of a shearing collar on the feed screw, and more particularly in communication with that portion of the interior of the cylinder wherein the plastic material is subjected to relatively low compressive stresses. The aperture extends upwardly from the interior of the cylinder, and the pressure of confined plastic material is regulated by axial shifting of the feed screw in such a way that the material does not tend to escape from the cylinder by way of the aperture. Such regulation is not possible in apparatus wherein the feed screw moves axially and rearwardly during each working cycle, i.e., wherein the feed screw moves axially for a purpose other than to regulate the pressure of confined material.
It is further known to allow for escape of gases from the plasticizing cylinder in the region where the granular material enters the cylinder. To this end, the material admitting or feeding unit is adjustable radially of the feed screw and its outlet portion defines with the inlet opening of the cylinder a gap for escape of gases. The means for adjustably mounting the feeding unit is complex because it should not prevent the feeding unit form admitting the material at a predictable rate and in such a way that the grooves between the threads of the feed screw are filled with material but the material does not pile up on top of the threads.